The perception of fog computing is to bring a virtual presence into day-to-day objects. The lowest layer of the fog architecture is the Internet of Things (IoT), which created a revolution by changing ordinary objects into smart objects that automatically sense and process data. In the IoT, smart objects connected over the Internet communicate with each other and exchange data with the fog server to improve services to customers. There are some challenges to achieving the benefits of the IoT. This article discusses a three-layered fog architecture and highlights potential security threats and solutions at each layer. Finally, open research issues are discussed at all three layers of the fog hierarchy.